Improved adaptive downlink for OFDM/SDMA-based wireless networks

A combined OFDM/SDMA approach substantially increases the capacity of wireless local networks while maintaining a feasible implementation complexity. In the downlink of OFDM/SDMA-based networks however, the standard channel inversion approach significantly boosts the transmit power. We introduce a low-complexity downlink OFDM/SDMA strategy, based on the combination of channel inversion and Tomlinson-Harashima (1971, 1972) pre-coding, which lowers the required transmit power by 4 dB. Additionally, adaptive loading of the subcarriers as a means for further lowering the required transmit power in the downlink of OFDM/SDMA networks utilizing the proposed pre-compensating strategy is studied. We derive an efficient closed-form expression for the optimal bit distribution and show that an additional 8.5 dB gain in transmit power can be achieved. Finally, a simplified loading algorithm is proposed which possesses a lower computational and signaling cost in return for a small performance penalty